The Math object
Learn rounding with Math.round, Math.floor and Math.ceil.
The Math object
In JavaScript Math is a built-in object that holds useful constants and methods for numbers. You do not create a new Math; you just use it directly.
Three common rounding methods are:
Math.round(x)— rounds to the nearest integer.Math.floor(x)— always rounds down to an integer.Math.ceil(x)— always rounds up to an integer.
For example Math.round(4.5) gives 5, Math.floor(4.9) gives 4 and Math.ceil(4.1) gives 5.
Down or up?
Math.floor and Math.ceil always go the same direction; that is why they serve different real world jobs.
When you compute remaining stock in a shop you use Math.floor: 7.8 boxes means 7 full boxes. But when you count the pages needed you use Math.ceil: even 3.1 pages means 4 pages, because you cannot print half a page.
The job, not the number, picks the rounding direction.
Exercise Round the score down and up
Declare const score = 7.8;. Then declare down with Math.floor(score) and up with Math.ceil(score). Log both with console.log.
Show example solution
const score = 7.8;
const down = Math.floor(score);
const up = Math.ceil(score);
console.log(down);
console.log(up);Test yourself
-
What does Math.round(4.5) return?
console.log(Math.round(4.5));- 4
- 4.5
- 5
- Throws an error
Show answer
5 — Correct. Math.round sends 0.5 and above upward; for 4.5 the result is 5.
-
Round 9.99 to the nearest integer
const rounded = Math.round(____);Show answer
const rounded = Math.round(9.99); -
Match each method to its job
- Math.round
- Nearest whole number
- Math.floor
- Down to whole number
- Math.ceil
- Up to whole number
- Math.PI
- The pi constant (~3.14)
-
What does Math.ceil(3.1) return?
console.log(Math.ceil(3.1));- 3
- 3.1
- Throws an error
- 4
Show answer
4 — Correct. Math.ceil always goes up; for 3.1 the next whole number up is 4.